1. Field of the Invention
The present invention relates to a plant control system.
2. Description of the Prior Art
The applicant of the present application has proposed an air-fuel ratio control system having an exhaust gas sensor for detecting the concentration of a certain component of an exhaust gas that has passed through a catalytic converter such as a three-way catalytic converter disposed in the exhaust passage of an internal combustion engine, such as an O.sub.2 sensor for detecting the concentration of oxygen in the exhaust gas, the exhaust gas sensor being disposed downstream of the catalytic converter. The system controls the air-fuel ratio of the internal combustion engine, more accurately, the air-fuel ratio of an air-fuel mixture to be combusted by the internal combustion engine, in order to converge an output of the O.sub.2 sensor, i.e., the detected value of the oxygen concentration, to a predetermined target value for enabling the catalytic converter to have a desired purifying ability irrespective of the aging of the catalytic converter. See, for example, U.S. patent application Ser. No. 09/311353, now U.S. Pat. No. 6,188,953 U.S. Pat. No. 6,112,517 and Japanese patent application No. 11-93740 (U.S. Pat. No. 6,079,205).
According to the disclosed technology, a manipulated variable for manipulating the air-fuel ratio of the internal combustion engine, specifically, a target air-fuel ratio or a quantity defining such a target air-fuel ratio, is successively generated in given control cycles in order to converge the output of the O.sub.2 sensor to its target value based on a feedback control process. An exhaust gas sensor (hereinafter referred to as an "air-fuel ratio sensor) for detecting the air-fuel ratio of the exhaust gas that enters the catalytic converter, specifically, the air-fuel ratio of the air-fuel mixture that has been burned by the internal combustion engine, is disposed upstream of the catalytic converter. The amount of fuel supplied to the internal combustion engine is regulated so as to converge the output of the air-fuel ratio sensor, i.e., the detected value of the air-fuel ratio, to a target air-fuel ratio defined by the manipulated variable for thereby controlling the air-fuel ratio of the internal combustion engine at the target air-fuel ratio.
Such air-fuel ratio control for the internal combustion engine is capable of converging the output of the O.sub.2 sensor disposed downstream of the catalytic converter to its target value for thereby enabling the catalytic converter to have a desired purifying ability.
In the above proposed air-fuel ratio control system, the feedback control process using the output of the air-fuel ratio sensor disposed upstream of the catalytic converter is carried out for controlling the air-fuel ratio of the internal combustion engine at the target air-fuel ratio. However, it is also possible to control the air-fuel ratio of the internal combustion engine at the target air-fuel ratio according to a feed-forward control process by determining the amount of fuel supplied to the internal combustion engine from the target air-fuel ratio using a map or the like.
In the above air-fuel ratio control system, the O.sub.2 sensor is used as the exhaust gas sensor disposed downstream of the catalytic converter. However, the exhaust gas sensor may comprise an NOx sensor, a CO sensor, an HC sensor, or another exhaust gas sensor. It is possible to enable the catalytic converter to have a desired purifying ability by controlling the air-fuel ratio of the internal combustion engine so as to converge the output of such an exhaust gas sensor to a suitable target value.
In the above conventional air-fuel ratio control system, the exhaust system, including the catalytic converter, which ranges from a position upstream of the catalytic converter to the O.sub.2 sensor downstream of the catalytic converter may be considered to be a plant for generating the output of the O.sub.2 sensor from the air-fuel ratio of the internal combustion engine (the air-fuel ratio as detected by the air-fuel ratio sensor). The internal combustion engine may be considered to be an actuator for generating an exhaust gas having an air-fuel ratio to be supplied to the plant. Thus, the air-fuel ratio control system may be expressed as a system for generating a manipulated variable to control the input (air-fuel ratio) to the plant (=an output from the actuator) to converge the output of the O.sub.2 sensor as the output of the plant to a given target value, and controlling operation of the internal combustion engine as the actuator based on the manipulated variable.
The catalytic converter generally has a relatively long dead time which tends to adversely affect the control process of converting the output of the O.sub.2 sensor to the target value. In the air-fuel ratio control system, the behavior of the exhaust system including the catalytic converter and ranging from the position upstream of the catalytic converter to the O.sub.2 sensor downstream of the catalytic converter is modeled. The output of the O.sub.2 sensor after the dead time of the exhaust system is successively estimated according to an algorithm constructed on the basis of the model of the exhaust system. The manipulated variable is generated using the estimated value of the output of the O.sub.2 sensor (specifically, the manipulated variable is generated in order to converge the estimated value of the output of the O.sub.2 sensor to the target value) for thereby compensating for the effect of the dead time.
In modeling the exhaust system including the catalytic converter, the exhaust system is regarded as a system for generating the difference between the output of the O.sub.2 sensor and its target value with a response delay and the dead time, from the difference (hereinafter referred to as a "differential air-fuel ratio") between the air-fuel ratio of the air-fuel mixture combusted by the internal combustion engine, i.e., the air-fuel ratio detected by the air-fuel ratio sensor, and a predetermined constant reference value with respect to the air-fuel ratio. The algorithm for estimating the output of the O.sub.2 sensor after the dead time of the exhaust system is constructed on the basis of the model of the exhaust system.
With the input (differential air-fuel ratio) to the exhaust system that is modeled and the output thereof (the difference between the output of the O.sub.2 sensor and its target value) being expressed as differences, the algorithm for estimating the output of the O.sub.2 sensor after the dead time of the exhaust system can be simplified.
The applicant has also proposed a technique for compensating for not only the dead time of the exhaust system, but also the effect of dead times of the internal combustion engine and a controller (hereinafter referred to as an "engine controller") which controls operation of the internal combustion engine based on the manipulated variable, in U.S. patent application Ser. No. 09/311353. This process is proposed because the dead times of the internal combustion engine and the engine controller may adversely affect the control process of converting the output of the O.sub.2 sensor to the target value, depending on an operating state of the internal combustion engine (more specifically, the state of the rotational speed thereof) for which the control process of converting the output of the O.sub.2 sensor to the target value is carried out.
According to the above proposed technique, the exhaust system is modeled as described above, and a system comprising the internal combustion engine and the engine controller is modeled as a system for generating an air-fuel ratio to be given to the exhaust system, with only the dead time of the air-fuel ratio control system, from the manipulated variable, i.e., a system in which the air-fuel ratio to be given to the exhaust system is uniquely defined by the manipulated variable prior to the dead time. According to an algorithm constructed on the basis of these models, the output of the O.sub.2 sensor after a total dead time representing the sum of the dead time of the exhaust system and the dead time of the air-fuel ratio control system is successively estimated, and the manipulated variable is generated using the estimated value. Therefore, it is possible to converge the output of the O.sub.2 sensor to the target value while compensating for not only the dead time of the exhaust system, but also the dead time of the air-fuel ratio control system.
In estimating the output of the O.sub.2 sensor after the total dead time, with the input (differential air-fuel ratio) to the exhaust system and the output thereof (the difference between the output of the O.sub.2 sensor and its target value) being expressed as differences, the algorithm for estimating the output of the O.sub.2 sensor can be simplified.
According to the above technique, a parameter for defining the behavior of the model of the exhaust system is successively identified using the output data of the air-fuel ratio sensor and the O.sub.2 sensor. The algorithm for estimating the output of the O.sub.2 sensor after the dead time of the exhaust system or the output of the O.sub.2 sensor after the total dead time which is the sum of the dead time of the exhaust system and the dead time of the air-fuel ratio control system uses the identified parameter of the model of the exhaust system. Furthermore, the above technique uses a sliding mode control process constructed on the basis of the model of the exhaust system as the feedback control process for generating the manipulated variable.
It has been found as a result of a further study by the inventors of the present application that when the output of the air-fuel ratio sensor suffers a steady offset from a normal output due to deterioration of the air-fuel ration sensor or when the actual air-fuel ratio is subject to a steady error with respect to the target air-fuel ratio due to an aging-induced characteristic change of the internal combustion engine, the estimated value of the output of the O.sub.2 sensor after the dead time of the exhaust system or the estimated value of the output of the O.sub.2 sensor after the total dead time which is the sum of the dead time of the exhaust system and the dead time of the air-fuel ratio control system tends to suffer a steady error with respect to a true value. If the estimated value of the output of the O.sub.2 sensor suffers such an error, then when the air-fuel ratio of the internal combustion engine is controlled on the basis of the manipulated variable generated using the estimated value, the accuracy with which to converge the output of the O.sub.2 sensor to the target value is lowered.
In addition, when the output of the air-fuel ratio sensor suffers an offset, the identified value of the parameter of the exhaust system model used for estimating the output of the O.sub.2 sensor is likely to suffer an error. Because of such an error, the estimated value of the output of the O.sub.2 sensor may be subject to an error.
Moreover, with the above process of generating the manipulated variable according to the feedback control process constructed on the basis of the model of the exhaust system, when the output of the air-fuel ratio sensor suffers an offset, the quick response of the control process for converging the output of the O.sub.2 sensor to the target value is reduced.
It has been desired to eliminate the above drawbacks.
In another application, a manipulated variable for controlling an input to an arbitrary plant is generated to converge a detected value of an output of the plant to a predetermined target value, and the operation of an actuator for generating the input to the plant is controlled on the basis of the manipulated variable. The above drawbacks are also caused in such an application when the output of an O.sub.2 sensor is estimated on the basis of a model of the plant which is constructed in the same manner as with the above technique in order to compensate for the dead time of the plant and the dead times of the actuator and its controller, and the manipulated variable is generated using the estimated value.